One crab passed away

One of my crabs passed away about a month ago. And I haven’t gotten another yet. Mostly because I wasn’t sure if the other one would pass as well, if I did something to cause it. I got them at the same time from the same pet store and they were about the same size. My newest struggle is I don’t think I want to continue to keep hermit crabs after this. I’ve been looking into a different type of reptile but in an apartment my fiancée said I can only have one.

Is it cruel to have one alone?

I, of course, don’t want my other one to pass but I don’t want him to be miserable alone.

Am I doing my other crab wrong by letting him live alone?